Currently in 3GPP, a Work Item to enhance the Physical Downlink Control Channel (PDCCH) for LTE is being discussed. In this Work Item, a new control channel, the Enhanced PDCCH (EPDCCH), is introduced. EPDCCH offers higher capacity for control channels and efficient use of resource via spatial reuse (MU-MIMO) and beamforming. EPDCCH shares the same resource space as that used by PDSCH (Physical Downlink Shared Channel).
An EPDCCH can be transmitted in a localized or distributed manner. Localized transmission schedules the resources for EPDCCH based on CSI (Channel State Information) reported by the UE on the resource space. The eNB selects the PRB (Physical Resource Block) pair with the desired radio condition to transmit the EPDCCH for the UE. Distributed transmission is usually used if no reliable CSI is available at the eNB. Here the EPDCCH is spread over several PRB pairs, so as to take advantage of frequency diversity. FIG. 1 shows an example of a localized transmission and a distributed transmission, wherein the smallest block stands for an Enhanced Resource Element Group (EREG) which will be described in the following. The term UE denotes User Equipment, and the term eNB denotes evolved NodeB, where a NodeB is a cellular telecommunications base station.
FIG. 2 shows the components of an EPDCCH. The building block of an EPDCCH is the Enhanced Resource Element Group (EREG), where an EREG consists of 9 Resource Elements (RE) that are distributed in a PRB pair. In localised transmission an EPDCCH is formed by at least one Enhanced Control Channel Element (ECCE) where an ECCE consists of several EREGs in the same PRB pairs. The total number of EREG per PRB pair is 16. The number of EREG in an ECCE is 4 or 8 (i.e. 4 or 2 ECCEs per PRB pair) depending on the amount of available RE in a PRB pair. In distributed transmission the EPDCCH consists of EREG from different PRB pairs. In the illustration of FIG. 2, the ECCE comprises 4 EREG in the same row, and the EPDCCH comprises 2 ECCE.
The number of ECCE (in localised transmission) and the number of EREG (in distributed transmission) in an EPDCCH is dependent upon the Aggregation Level (AL) of the Downlink Control Information (DCI) message carried by the EPDCCH. The higher the AL is, the more ECCE (or EREG) is required in the EPDCCH. The amount of ECCE is equals to the AL, that is if the AL is 2 then 2 ECCEs (or the equivalent number of EREGs) are required to form the EPDCCH. The AL for localised transmission can be 1, 2, 4, 8 and 16 whilst the AL for distributed transmission can be 2, 4, 8, 16 and 32.
The AL and the ECCE/EREG containing the EPDCCH for a UE is not signalled to the UE. Instead, each UE is configured with a search space which consists of possible ECCE/EREG and AL combination candidates for an EPDCCH. The UE performs blind decoding on all the possible candidates to search for the one that may contain an EPDCCH intended for it. The search space consists of K EPDCCH sets where each set has NPRB PRB pairs that can contain EPDCCH. The K EPDCCH sets consist of KL sets for localised transmission and KD sets for distributed transmission.
Blind decoding consumes UE processing power and increases UE complexity. Therefore the number of blind decodings is not expected to exceed 32. However, the possible number of candidates (i.e. blind decodings) in a search space can be very large given the possible combinations of AL and parameters NPRB and K of the search space. Consequently there is a need to limit this number of candidates. However, a reduction in the number of candidates reduces the scheduling flexibility of the eNB.